1. Field of the Invention
This invention relates to polymerizable, curable compositions. More specifically, it relates to molding compositions comprising an unsaturated polyester resin and the reaction product of a polyepoxy compound with an unsaturated monocarboxylic acid. Still more specifically, it relates to such compositions in which at least 5 percent of the original epoxy groups remain free or unreacted at the time of mixing with said polyester.
2. State of the Prior Art
In the prior art of unsaturated polyester technology, new formulation studies have been made in the development of solid molding compounds. An important feature is continuous processing and utilization via injection and transfer molding.
In order to prepare solid powder, or granular molding compounds, a solid unsaturated polyester resin has to be mixed with a solid or liquid reactive monomer. In the first case, the formulations are often limited by the availability of solid reactive monomers. Also, the use of a solid monomer in a matrix system can sometimes impose a limitation on the loadability of the matrix system, in particular, for some very finely divided reinforcing fillers. For instance, when the melting temperature of the matrix system is very close to the curing temperature or the molding temperature of the system, the wettability of such fillers is often found inadequate in such matrix system, and the product made from such composition shows poor resistance to thermal or humid aging.
The use of low boiling liquid vinyl monomer, such as styrene, as crosslinker is common in the production of sheet molding and bulk molding compounds. Even though this polyester/vinyl monomer system cures fast and provides good properties, it does have several drawbacks in that the vinyl monomer is often very volatile under processing conditions. In addition, the exotherm temperatures during curing are quite high, which often results in micro-cracks in the final product. It is also believed in the art that the shrinkage of the cured product is inversely proportional to the molecular weight of these vinyl monomers. It is thus advantageous to use a high molecular weight reactive monomer, with higher boiling temperature. In particular, the use of low boiling liquid, reactive vinyl monomer, such as styrene, is often found inadequate in making solid powder or granular molding compounds.
A number of patents have shown the use of the reaction product of a polyepoxy compound, such as the diglycidyl ether of bisphenol-A, with acrylic or methacrylic acid in combination with an unsaturated polyester resin to produce a matrix resin. However, in each of these patents, it is indicated, either specifically or by the excess of acid used, that this reaction product has substantially no free or unreacted epoxy groups when this product is added to the unsaturated polyester resins.
These patents are: Netherland Pat. No. 67-10103 (issued in 1969 to H. H. Robertson Company); and U.S. Pat. Nos. 3,506,736, 3,621,093, 3,825,517 and 3,968,016. As stated above, none of these references indicate that the polyepoxy reaction product with acrylic or methacrylic acid has any amount of free or unreacted epoxy groups.